REDUCING THE RISK FROM COASTAL GEOLOGIC HAZARDS

U.S. Geological Survey
Fact Sheet 150-00
2000

Helping Coastal Communities at Risk from TsunamisThe Role of U.S. Geological Survey Research

In 1946, 1960, and 1964, major
tsunamis (giant sea waves
usually caused by earthquakes
or submarine landslides) struck
coastal areas of the Pacific
Ocean. In the U.S. alone, these
tsunamis killed hundreds of
people and caused many tens of
millions of dollars in damage.
Recent events in Papua New
Guinea (1998) and elsewhere are
reminders that a catastrophic
tsunami could strike U.S. coasts
at any time. The USGS, working
closely with NOAA and other
partners in the National Tsunami
Hazard Mitigation Program, is
helping to reduce losses from
tsunamis through increased hazard
assessment and improved
real-time warning systems.

Aftermath of the 1960
Chilean tsunami in
Hilo, Hawaii, where
the tsunami caused
61 deaths (photo
courtesy Pacific
Tsunami Museum).
Inset shows damage
from a local tsunami
that struck Okushiri
Island, Japan, in 1993
(photo by Dennis
Sigrist, International
Tsunami Information
Center). Floating
debris, such as seen
in these photographs,
can cause
significant
destruction
during
tsunami
runup.

Population pressures in the coastal zones
of the Pacific Ocean and the Caribbean are
resulting in unprecedented shoreline development,
putting residents, tourists, and
property at increased risk from giant sea
waves, called tsunamis. Tsunamis, often incorrectly
referred to as “tidal waves,” can be
generated by distant earthquakes and by local
seismic events, submarine landslides,
and volcanic eruptions. For ocean-crossing
tsunamis, there is often sufficient time to
evacuate distant coastal areas, but more
timely and accurate real-time tsunami forecasts
are needed to avoid costly false
alarms. Local tsunamis generated by
quakes on active seismic zones in Alaska,
the Pacific Northwest, the Caribbean,
California, and Hawaii can arrive at
nearby shorelines in minutes. In these
cases, only better scientific understanding,
informed disaster planning, and public
education will save lives in future tsunamis.
Protecting lives and property from tsunamis
demands a clear understanding of how
tsunamis are generated, the identification of
likely areas at risk, and mitigation efforts
based on public education. The National
Tsunami Hazard Mitigation Program
(NTHMP), a partnership of the States of
Alaska, California, Hawaii, Oregon, and
Washington and the Federal Emergency
Management Agency, National Oceanic and
Atmospheric Administration (NOAA), and
U.S. Geological Survey (USGS), is currently
preparing tsunami inundation maps
and implementing mitigation plans for
states bordering the Pacific Ocean. The
NTHMP is also providing tsunami early
warnings for these states by means of deepocean
tsunami detectors and new seismic
stations on land. The cooperation of Federal
and state agencies in the NTHMP is
furthering scientific understanding of tsunami
hazards and facilitating the development
of plans to prepare coastal communities
to better survive future tsunamis.
The mission of the USGS includes understanding
the geologic mechanisms, frequency,
magnitude, and physical consequences
of natural hazards. In recent years,
the USGS has upgraded its seismic networks
to provide NOAA's tsunami warning
centers with detailed earthquake information
to aid in timely notification of the public.
The USGS and NOAA are currently
providing real-time seismic and tsunami information
to state offices of emergency services
in Alaska, Washington, Oregon, California,
and Hawaii.

To ensure that its efforts to reduce the risk
from tsunamis are focused where they will
be most effective, the USGS convened a
two-day workshop in Seattle, Washington,
in January 2000. This workshop was attended
by scientists and managers from the
USGS, NOAA, state agencies, and
academia. Based on needs identified by the
workshop participants, the following recommendations
on the role of the USGS in tsunami
research were made:

Seismic networks—It is important
that, as advances in technology and science
allow, the USGS continue to enhance
and improve the quality and quantity of
seismic data supplied to tsunami warning
centers. The USGS should also determine
the need for additional strong-motion seismic
stations along U.S. coasts. Further, the
USGS should continue to develop software
that supports tsunami warnings for local
earthquakes, particularly in Hawaii, Alaska,
the Pacific Northwest, and the Caribbean.

A tsunami caused by an earthquake along a
subduction zone happens when the leading edge
of the overriding tectonic plate breaks free and
springs seaward, displacing the sea floor and the
water above it. The arrival time and severity of the
tsunami can best be predicted using complex
earthquake parameters, such as accurate depth
determination, focal mechanism, and highresolution
sea-floor displacement patterns.

Earthquake source characterization—
In order to provide accurate warnings and
hazard assessments, better characterizations
of critical earthquake source parameters are
needed. Modern methods of analyzing seismic
data developed by the USGS can be
used by tsunami warning centers in an overall
effort to improve the accuracy and timeliness
of tsunami warnings. Further research
is also recommended into using rapid seismic
inversion algorithms to estimate seafloor
displacement, an indicator of likely
tsunami size, which could be automated as
part of local tsunami warning systems.

Hazard assessments—The NTHMP is
coordinating the preparation of tsunami inundation
maps for high-risk coastal communities
in Alaska, California, Hawaii, Oregon,
and Washington. The USGS can provide
valuable guidance in the preparation of these
maps by analyzing and interpreting deposits
from historic and prehistoric tsunamis to estimate
inundation limits, flow velocities, and
recurrence intervals.

Coastal bathymetry and topography—
Producing reliable and useful inundation
maps for hazard assessment partially depends
on accurate bathymetry and topography
of coastal regions. The USGS, along
with NOAA, NASA, and the Army Corps of
Engineers, have proven capability to survey
coastal and nearshore bathymetry and topography
using modern techniques. It is
recommended that the USGS coordinate
with other Federal agencies to provide
such information for hazard mitigation
planning in high priority areas of the Pacific
Northwest, Alaska, Hawaii, California,
and the Caribbean.

Sedimentary deposits as keys to tsunami
character—Identification and interpretation
of sedimentary deposits left
behind by prehistoric tsunamis will improve
our ability to assess the magnitude
of tsunami risk in areas with an insufficient
historical record. The USGS will
provide expertise and leadership in sediment
transport modeling to increase understanding
of the mechanics of sediment
transport in tsunamis and of the deposits
they leave behind. Research would occur
in field settings by integrating predictive
modeling with laboratory and post-event
studies.

Post-event rapid response—The visible
effects of tsunamis are short-lived
and may be lost after a single subsequent
storm or during clean-up efforts employing
earth-moving equipment. To determine
the effects of tsunami inundation on
land, the run-up elevation and distance,
flow-speed and direction indicators, and
patterns of sedimentary deposition must
be mapped and quantified immediately
following an event. USGS scientists
should join the International Tsunami
Survey Team, when warranted, to gather
information about tsunami deposits and
to calibrate sediment transport models.

Computer simulation of a major nearshore earthquake
along the Cascadia Subduction Zone off the
Pacific Northwest shows the advancing wave fronts
of the resultant tsunami.

Tsunamis generated by landslides
and volcanic events—Landslides and
volcano flank failures in coastal and island
settings have also initiated large tsunamis.
The accumulated knowledge of
USGS scientists about submarine and
coastal landslides and active volcanic
processes should be focused on improving
understanding of how and where these catastrophic
mass failures may occur. This information
then can be used in regional
hazard assessments by Federal, state, and
local authorities.

The efforts of USGS and other cooperators
in the NTHMP are leading to a better
understanding of tsunamis and how
coastal populations can be prepared to survive
their onslaught. The work of USGS
scientists in tsunami research is only part
of the ongoing efforts of the USGS to protect
people's lives and property from geologic
and environmental hazards in the
coastal zones of the United States.

NATIONAL TSUNAMI HAZARD MITIGATION PROGRAM
States of Alaska, California, Hawaii,
Oregon, and Washington
National Oceanic and Atmospheric Administration
Federal Emergency Management Agency
U.S. Geological Survey